One conventional omnidirectional air speed sensing system uses a pair of pressure sensing ports disposed within shrouds mounted on the distal ends of diametrically opposed arms rotatable about their central axis. A pressure transducer at the conjunction of the arms senses the differential pressure at the two pressure ports and converts that to an electrical signal which is transmitted along the rotating shaft through slip rings to the electronic circuits which determine the air speed from the differential pressure signal. By using two spaced pressure sensing ports, that system inherently pneumatically cancels signals such as the d.c. component, and second harmonic and higher harmonics which are not essential to determining air speed and leave a single, first harmonic, linear in nature, which represents by its amplitude, the air speed, and by its phase, the sense of direction of the aircraft. The elongate arms which support the pressure ports serve to increase the magnitude of the first harmonic and thus insure higher signal-to-noise ratios and more accurate speed readings.
The elongate rotatable arms tend to make the device large, cumbersome and delicate, as well as difficult to place on aircraft. Since the two pressure ports contribute to the cancellation of unwanted signal components, the arms, ports and shrouds must be carefully balanced and mirror each other to a high degree to ensure that the final signal output is clean and accurate. If the differential pressure transducer is in the rotating part then the electrical signal must be transmitted from the rotating to the non-rotating part using slip rings or the like, which add expense as well as maintenance and reliability problems. If the differential pressure transducer is in the non-rotating part then there are two pressures that have to be conducted from the rotating to the non-rotating part.
With the introduction of a technique for extracting the fundamental or first harmonic from the d.c. component and higher harmonics, as taught in copending application filed on even date herewith, entitled "Harmonically Filtered Speed Sensing System", Joseph R. Fisher, the use of a single and shorter arm has been achieved. However, many of the shortcomings associated with two arms and longer arms, such as bulkiness, large size, fragility and difficulty in placement, still persist.